1. Field of the Invention
The invention relates to a rolling probe for the continuous measuring of the thickness of layers or strips.
2. Background Art
Rolling probes for the continuous measuring of the thickness of layers or strips of electrically conducting or insulating, magnetic or nonmagnetic material, or of layers of such strips whose material properties differ from those of the carrier strip, with a measuring pole, to which a stationary sensor system is allocated, are known.
In such a known measuring probe (West German Patent No. 3,401,466), the measuring pole is placed inside of a hollow cylinder arched spherically in the area of the measuring pole and directed perpendicular to the strip or to the layer to be measured, and the cylinder, rolling off, contacts the strip or the layer only at one point of support and consists of a nonmagnetic material that does not significantly influence the measuring.
In this measuring probe, a magnetic air gap for design reasons is between the active measuring pole and the contact surface of the layer to be measured, an air gap whose size depends on heat expansions because of temperature influences and by which changes of measured values also are possible in a tilting of the measuring pole in traversing operation. Also, material inhomogeneities in the measuring roller can cause incorrect measured values, and the error increases with the magnetic air gap growing larger between an active sensor or measuring pole and the surface to be measured.
Also, a maximum use of the measuring signal is not possible since it is reduced by the size of the magnetic air gap. Further, incorrect measured values can be caused by eddy-current effects in the high-grade steel material of the roller and, further, the maximum requirements of the concentricity tolerance of about 0.002 mm of the roller can be met only with difficulty relative to the fixed axis of rotation of the roller. Also, an easy deformability of the roller is produced by the small wall thickness in the area of the contact of the hollow roller with the surface of the object to be measured. Such rollers also require a high production expense.
Difficulties relative to the measuring accuracy of rolling probes also occur when the measuring pole is placed between two ball races contacting the layer to be measured. In such a measuring probe, the magnetic air gap between the measuring pole and the object to be measured can be kept comparatively small to better use the available measuring signal. But the measuring does not take place at the point of contact, since the two ball races are placed as pure idlers laterally or in tandem on both sides of the intervening measuring pole.
Also, in such a rolling probe, the measuring point and the contact point are not identical, by which a measured value is detected which does not have to correspond to reality.
Such a two-point sensor also has to be mounted on gimbals to assure that both idlers always satisfactorily contact the layer to be measured. But a traversing operation is not possible with a suspension on gimbals. The sensor or measuring pole tilts, and incorrect measurements result. Also, sheets which are more narrow than the distance of the parallel idlers cannot be measured.
In measurements which are to be performed on the sheet edge, which is bulge-like for production reasons, incorrect measurements also result by different sheet thicknesses under both idlers.
Also known in the patent literature is a two-point-rolling probe, i.e., an electromagnetic instrument for measuring the thickness of nonmagnetic layers on ferromagnetic bases, for example, of paint layers on iron. It can also be applied on nonmagnetic strips, sheets, etc., for thickness measurement if they lie on an iron base or run over an iron base.
Electromagnetic thickness gages are known, whose measuring probe is an iron core with a winding applied on it. The winding flowing through by an alternating current changes its impedance when approaching a ferromagnetic surface, which causes the deflection of an electrical measuring instrument, which can be calibrated in units of thickness of a layer.
The two-point-rolling improves the sensitivity of such a thickness gage. It is characterized in that the measuring probe consists of an H-shaped ferromagnetic core, an exciter winding is placed on its cross bar, and a measuring winding each on its four sides, which are electrically connected in series. The winding direction of the individual measuring windings is selected, so that an electrical measuring voltage is produced in the windings facing the layer to be measured, which is opposite of the voltage of the two other measuring windings facing away from the layer to be measured. The resultant differential voltage is a measurement for the distance to be measured of the probe from the ferromagnetic layer or for the thickness of a nonmagnetic layer located between the two.
The sides of the H-shaped measuring probe according to the invention can also be connected securely with the cross bar, can have a rectangular or any other cross-section or be matched in a suitable way to the shape of the parts to be measured. In this connection it is also suitable for disk-like sides to be securely connected with the cross bar and be placed rotatably in a coil shape, which supports the exciter winding and the measuring windings.